This invention relates generally to radio communication receivers, and more specifically to a radio communication receiver incorporating DC offset correction loops for Zero IF or Direct Conversion architectures.
DC offset receivers and Zero IF receivers are discussed generally in U.S. Pat. No. 4,653,117 issued in March 1987, to Heck, U.S. Pat. No. 5,079,526 issued in January 1992, to Heck, et al.; in U.S. Pat. No. 5,483,691 issued in January 1996, to Heck, et al.; in U.S. Pat. No. 5,893,029 issued in April 1999 and in U.S. Pat. No. 6,006,079 to Jaffee et al. issued in December 1999, which are hereby incorporated by reference.
Direct Conversion Receivers (DCR) or Zero IF (ZIF) architectures function by mixing the desired RF or IF signal down to baseband, or some very low frequency offset from DC. Therefore, by definition for DCR or ZIF receivers, the mixer""s Local Oscillator (LO) frequency is approximately equal to the desired input RF frequency. Thus, the magnitude of the baseband DC signal is proportional to that portion of the RF signal that is exactly equal to the LO frequency. Any variations in RF power due to environmental (fading, multi-path) or circuit functionality (AGC, mixer LO-to-RF isolation) will affect the Direct Current (DC) voltage level at baseband. It is important for the optimum performance of the receiver that variations in the baseband DC are compensated. This implies a need for a Direct Current Offset Correction Loop (DCOCL) which can unobtrusively compensate for any DC variations in the baseband signal path.
The baseband signal path may include a parallel I and Q channel configuration, where the Q signal is 90xc2x0 out of phase with 1. Great effort is usually expended to maintain symmetry between the I and Q channel circuitry in order to minimize distortion products that result from amplitude or phase imbalances between the respective paths. This is especially true for analog modulation schemes, where distortion products cannot be eliminated through DSP arithmetic manipulation such as forward-error correction, or auto-correlation techniques. Each I and Q channel often includes a differential baseband path (I and {overscore (I)}, Q and {overscore (Q)}), where I/{overscore (I)} and Q/{overscore (Q)} maintain a 180xc2x0 phase relationship to enhance common-mode noise immunity. Any DC offset between I and {overscore (I)}, or Q and {overscore (Q)} signals, is interpreted as a shift in the intrinsic DC for the I and Q channels respectively.
If the DC shift is severe enough, any demodulation technique (digital or analog) which requires an accurate reference for the I and Q signals will be degraded. Furthermore, severe DC shifts within the baseband path can impact circuit performance, degrading the selectivity of the baseband active filters, minimizing the AGC-free dynamic range, and even xe2x80x9crailingxe2x80x9d the I/Q signal against the baseband bias limits. This can be further complicated by the finite isolation between RF and Local Oscillator (LO) signals in the down mixers of many real world Zero IF (ZIF) or Direct Conversion Receivers (DCR). This finite isolation can cause the LO to mix with itself creating a baseband voltage proportional to the LO to RF isolation. This phenomenon has historically complicated ZIF design implementations since baseband DC offsets created in this manner vary with mixer performance over temperature, gain and LO drive level.
The continuously tracking closed loop offset correction strategies (which continuously track out DC variations) afford the advantage that all variations in DC voltages are continuously tracked out in real time, continuously xe2x80x9ccenteringxe2x80x9d the I/Q signal thus providing maximum AGC-free dynamic range of the baseband path. The disadvantage of a continuously tracking closed loop strategy is that it effectively introduces a High Pass Filter (HPF) response into the baseband filter response by tracking out all baseband signal variations below the DCOCL loop bandwidth. This high pass filter response induces an equivalent xe2x80x9cnotchxe2x80x9d in the ZIF equivalent passband. The notch effectively xe2x80x9cnulls outxe2x80x9d FM Bessel components of the desired RF signal that are equal to the local oscillator frequency, thus causing distortion in the time domain demodulated signal. While these distortion products could easily be accommodated in digital modulation protocols, constant envelope analog modulation (FM, PM, etc) is seriously distorted by this xe2x80x9cnotch effectxe2x80x9d.
U.S. Pat. No. 5,079,526 by Heck et al, attempts to address this condition by phase-locking the LO (the LO defines the notch location in the ZIF pass band) to the RF signal at a known LO-to-RF offset. The offset is selected to minimize any distortion products; however, the notch still exists and it becomes very cumbersome to effectively optimize the LO-to-RF offset for all operating environments.
U.S. Pat. No. 5,483,691 by Heck et al. optimizes AGC performance in a ZIF system to the exclusion of integrating the functionality into secondary loops unrelated to AGC. So, a need exists for an xe2x80x9coptimizedxe2x80x9d digital DCOCL methodology integrating AGC and DC offset correction functionality that can be initiated on an xe2x80x9cas requiredxe2x80x9d basis. Such a method will provide elimination of a passband notch while maintaining an accurate baseband DC voltage. This in turn provides optimum performance for both analog FM and digital protocols.
Thus, there is a need for a baseband DC offset correction method which can be universally applied to ZIF and Direct Conversion receivers in a manner which is minimally disruptive of normal communications without the negative effects of the characteristic ZIF passband notch produced by continuously tracking out DC offset variations and without the RF to LO isolation issues of previous designs.